Due to the inherent characteristics of certain crystals, they can be made to oscillate at a very precise frequency. Thus, crystal controlled oscillators are often used in applications where a precise frequency is required.
The crystals used to control crystal oscillators behave like a resonant circuit that contains an inductor, a capacitor and a resistor. That is, when a transient signal is applied to a crystal, it oscillates similar to the manner that a resonant circuit oscillates.
Crystal oscillator circuits operate by taking a signal from a crystal, amplifying that signal and feeding the signal back to the crystal to sustain (or increase) the crystal's oscillation. When power is initially applied to a crystal oscillator circuit, random thermal noise, or other random transient signals, initiate oscillations in the crystal. The oscillations grow over time and finally they reach a normal or steady state value. Typically a crystal oscillator takes in the neighborhood of 20,000 to 30,000 cycles to settle into a final amplitude.
FIG. 1A illustrates a prior art crystal oscillator. The oscillator illustrated in FIG. 1A includes a single pin oscillator circuit 10 and a crystal 11. When circuit 10 is powered on, thermal noise or some other type of random transient signal causes the crystal 11 to begin oscillating, the oscillations in crystal 11 are amplified by the single pin oscillator circuit 10 and fed back to the crystal 11. The signal at the terminals of crystal 11 grows as illustrated in FIG. 1B. In FIG. 1B, the horizontal axis is time in nanoseconds and the vertical axis is micro volts of output at the crystals terminals. It is noted that for ease of illustration, FIG. 1B only illustrates a limited number of cycles; however, a substantial number of cycles may be required (typically in the range of 20,000 to 30,000 cycles) before the output signal reaches the normal operating range.
In some applications, the amount of time required to power up and stabilize an oscillator is of great concern. The circuitry described herein reduces the amount of time required to initiate the stable operation of a crystal oscillator.